The electroless tin-plating of copper workpieces on the outside by means of an aqueous tin-plating solution is a common process in surface-coating technology. It is used, for example, for tin-plating the inside of copper pipes or tin-plating printed circuit boards for integrated circuits.
The tin-plating solution contains aqueously dissolved tin ions that are deposited on the copper by chemical reduction using a suitable reducing agent. In doing this, an exchange between the metals takes place at the surface of the copper workpieces, which is made possible by a complexing agent contained in the tin-plating solution. Hypophosphite is used primarily as the reducing agent and thiourea is typically used as the complexing agent.
By lowering the redox (oxidation-reduction) potential of copper in the coordinated form, copper goes into solution and tin deposits on the surface of the copper workpiece. Since no free electrons appear during this type of chemical reaction, the oxidation of one reaction partner is always accompanied by the reduction of another.
Consequently, an enrichment of copper and a depletion of tin in the tin-plating solution is associated with the process of electroless tin-plating. Therefore, in conventional operation, the tin and the complexing agent must be regenerated until a limiting concentration of copper is reached, at which point the solution is unusable and must be replaced. In addition, the reducing agent must be regenerated from time to time, since it is expended when, after achieving a complete tin coating, further metal still needs to be deposited.
The exhausted tin-plating solution then contains tin and copper ions, free complexing agent and complexing agent bound to the copper ions, expended and unexpended reducing agent, and possibly other constituents subject to the process technology.
To regenerate a galvanic tin-plating electrolyte, DE 27 42 718 A1 proposes removing the tin ions first of all by means of electrolysis and then, subsequently, removing the foreign-metal ions in a cation exchanger.
Regarded as related art through DE 43 10 366 C1 is a method and device for regenerating aqueous coating solutions, working with zero current on the outside, for metal coating by means of metal ions and a reducing agent. In this case, an ion-exchange process is carried out in combination with the electrolytic electrode reactions.
The process takes place in an electrolytic cell having at least four chambers. Electrolytic regeneration is achieved during the process by reducing orthophosphite to hypophosphite in a cathode chamber and by electrodialytic provision of counterion-free regenerating chemicals.
Electrolytic regeneration of tin-plating solutions, working in an electroless manner on the outside, could not be practiced successfully till now, since the thermodynamic potentials of the coordinated copper and tin tend to prohibit such copper deposition.